Dewatering saturated, networked suspensions with a screw press

Abstract

A model is presented for the dewatering of a saturated two-phase medium in a screw press. The model accounts for the detailed two-phase rheological behaviour of the pressed material and splits the press into two zones, an initial well-mixed constant-pressure region followed by an axial transport region in which the total pressure steadily increases. In this latter region, a slowly-varying helical coordinate transformation is introduced to help reduce the dynamics to an annular bi-axial compression of the two-phase medium. Unlike previous modeling, the transition point between the two zones is determined self-consistently, rather than set a priori, and the pressure along the length of the press is deduced from the rheology of the two-phase flow rather than averaging the two-phase dynamics over a cross-section of the press. The model is compared to experimental observations of the dewatering of a paper-making fibre suspension and of a clay slurry, and is shown to reproduce operational data.

On two‐phase modeling of dewatering pulp suspensions

“…Pulp suspensions show very limited recovery on unloading, and so we interpret the rate-independent solid stress as a compressive plastic yield stress. Following previous work, 15,16,27,28,32,45 we have included a rate-dependent component to the solid stress with the form of a bulk viscosity to gauge whether this addition improves theoretical modeling of the dewatering behavior of the entire pulp library. The library consisted of a range of different source woods and preparation and refinement techniques, which we split into three broad "families": chemical pulps with variant degrees of chemical, but not mechanical, refinement; mechanically refined chemical pulps; and mechanical pulps (with or without subsequent mechanical refinement).…”

“…To determine the bulk viscosity function η ( ϕ ), we first note that some success in reproducing the dewatering dynamics of pulp suspensions has been achieved using the functional form, where the constant η * = O (10 7 )Pa ⋅ s 15,27,45 . We continue with this form here, and estimate the parameter η * for each pulp from our experimental data as follows.…”

“…where the constant η * = O(10 7 )Pa Á s. 15,27,45 We continue with this form here, and estimate the parameter η * for each pulp from our experimental data as follows. Given the load σ Exp (t; V) averaged over each of the four duplicate trials for each compression speed V, we minimize the objective error function ℱ,…”

Modelling of dewatering wood pulp in a screw press using statistical and multivariate analysis

“…Attempts to model the screw press dewatering have been based on dividing the press into two zones (Shirato et al 1985;Egenes and Helle 1994;Eaves et al 2020). First, the material is dewatered by filtration and moving it along the press to a point where the material starts to become more solid.…”

“…At this point, there is a pressure build-up, where the dewatering by compression takes over. The models developed by Eaves et al (2020) are based on two-phase flow formulations by using Darcy's law for the fluid and the effective solid network stress for the solid part. In their paper, they defined the transition point from six central points, giving 32 experiments.…”